Polyesters have been used for various purposes such as fibers, films and containers, because of their excellent mechanical properties and moldability. In particular, polyarylates which are polycondensates of aromatic dicarboxylic acids and aromatic diols exhibit various excellent properties (such as heat resistance, flame retardance, impact resistance, bending recovery properties, UV barrier properties, chemical resistance and electric properties), in addition to the mechanical properties and moldability, so that they have been widely used as engineering plastics in many industrial fields.
As methods for producing polyarylates, there have been known, for example, the following methods:
(i) An interfacial polycondensation method of an alkali metal salt or alkali earth metal salt of an aromatic diol compound with a dicarboxylic acid dichloride,
(ii) An acetic acid elimination polycondensation method (melt polycondensation method) of a diacetate of an aromatic diol compound with a dicarboxylic acid, and
(iii) A phenol elimination polycondensation method (melt polycondensation method) of a diphenyl ester of a dicarboxylic acid with an aromatic diol compound.
However, the method of (i) has problems that the expensive dicarboxylic acid dichloride is used as a raw material, that a solvent such as methylene chloride is used in large amounts, that it is necessary to wash and remove a salt generated in the reaction, that impurities such as a solvent remain in the polyarylate, and the like.
In the methods of (ii) and (iii), raw materials can be synthesized from various dicarboxylic acid derivatives, but the reaction is performed at high temperature. Therefore, coloration is liable to occur due to the influence of oxygen and the like. Further, the melt viscosity of the reaction system becomes extremely high at the late stage of the reaction, so that it becomes difficult to efficiently draw out reaction by-products (such as water, monocarboxylic acids and phenols) to the outside of the reaction system. Accordingly, the polycondensation reaction for increasing the molecular weight to a desired value is required to be performed at high temperature for a long period of time. Furthermore, if the monocarboxylic acids or the phenols remain in the polyarylate, they cause coloration or strength reduction of the polyarylate.
As methods for drawing the monocarboxylic acids or the phenols out of the polyarylate of a high-viscosity state in the melt polycondensation method, there have been proposed, for example, the following methods:
(iv) A method of continuously reacting a diphenyl ester of a dicarboxylic acid with an aromatic diol compound by using a combination of a wiped film reactor and a twin-screw horizontal extruder having 5 exhaust outlets (see Patent Document 1),
(v) A method of polymerizing a polyarylate prepolymer in a molten state while allowing it to freely drop from a perforated plate (see Patent Document 2), and
(vi) A method of polymerizing a polyarylate prepolymer in a molten state while allowing it to drop along a guide from a perforated plate (see Patent Document 3).
However, the methods of (iv) to (vi) also have a problem that the temperature of the polycondensation reaction must be set high, in order to draw out the phenols having a high boiling point to the outside of the polymerization system.